Populations and ecosystems: WJEC A level Biology notes

Lesson 1 of 19

How do populations grow and change in size?

These notes cover topic 3.5, size and , in WJEC A level Biology Unit 3: Energy, and the Environment. Each lesson follows the WJEC specification, and the exam tips are based on BioWelsh analysis of WJEC past papers.

A is a group of organisms of one , living in the same place at the same time, that can interbreed. The rabbits in one field, the barnacles on one rocky shore and the yeast cells in one flask are all .

Only four things can change the size of a . Births and (individuals moving in) add individuals. Deaths and (individuals moving out) remove them.

When a few individuals arrive in a new with plenty of resources, the grows in a typical pattern. Growth is slow at first, then very fast, then it levels off. The same pattern is seen in bacteria growing in a flask. (For the bacterial version, read the Microbiology lesson on bacterial growth curves.)

The graph below shows the phases of growth of a in a closed environment, where no new resources are added.

Line graph of number of individuals in thousands against time from 0 to 40 days. The S-shaped curve starts near 0, rises slowly to about 3 thousand at 5 days, rises steeply through 50 thousand at about 13 days, levels off at about 89 thousand between 22 and 30 days, then falls to about 58 thousand at 40 days. Labels read lag phase near the start, exponential phase beside the steep section, stationary phase above the plateau and death phase beside the fall. — The four phases of growth of a population in a closed environment. Original BioWelsh graph.

: numbers rise slowly. There are few individuals to reproduce, and they are still adjusting to the new environment, for example by making or reaching breeding age.
: numbers rise faster and faster. Resources are plentiful, so is much greater than death rate, and each generation is larger than the last.
: the curve levels off. Resources become limiting, so falls and death rate rises until they are equal.
: numbers fall. Nutrients run out and toxic waste builds up, so death rate is greater than . This happens in a closed culture; wild usually fluctuate instead.

produces very large numbers, so microbiologists often plot the of the number of cells. On a , each unit on the axis is ten times the one below, and appears as a straight line.

The graph below shows a bacterial plotted on a . The dashed lines mark the start and end of a straight section.

Line graph of log base 10 number of bacteria per cubic centimetre, from 1 to 7, against time from 0 to 12 hours. The line is flat at 2.5 until 1 hour, curves up to 3 at 3 hours, rises in a straight line to 5 at 7 hours, then curves and levels off towards 6 by 12 hours. Dashed lines mark the value 3 at 3 hours and 5 at 7 hours. — Growth of a bacterial population plotted on a log₁₀ scale. Original BioWelsh graph.

Between 1 and 3 hours the line curves upwards slowly: this is the .
Between 3 and 7 hours the line is straight, so the is growing .
At 3 hours the value is 3, so there are 10³ = 1 000 bacteria per cm³. At 7 hours the value is 5, so there are 10⁵ = 100 000 bacteria per cm³.
After 7 hours the line levels off as the enters the .

Lesson 2 of 19

What is carrying capacity and what limits population growth?

No can grow for ever. Sooner or later a resource runs short: food, water, light, space, nesting sites or oxygen. The largest that an environment can support for a long time is called its .

All the factors that stop a growing are together called . Some are (caused by living things), such as competition, predation, parasites and disease. Others are (non-living), such as temperature, rainfall, light and pH.

The graph below shows a that grows until it reaches the of its .

Line graph of number of individuals from 0 to 100 against time from 0 to 50 years. The curve rises slowly, then steeply between 5 and 13 years, and approaches a dashed horizontal line at 80 labelled carrying capacity. From about 18 years the curve oscillates regularly above and below the line, between about 74 and 86. — A population growing to its carrying capacity and then fluctuating around it. Original BioWelsh graph.

At first the grows , because is low.
As numbers approach the dashed line, increases and growth slows.
After about 18 years the rises above and falls below the in a repeating pattern.
The is never exactly at the ; it fluctuates around it.

These fluctuations are an example of , shown in the diagram below.

Cycle diagram of four boxes joined by arrows. Population increases leads to more competition, predation and disease, labelled environmental resistance rises. This leads to population decreases, which leads to less competition, predation and disease, labelled environmental resistance falls, which leads back to population increases. The centre reads population size fluctuates around the carrying capacity. — Negative feedback keeps a population close to its carrying capacity. Original BioWelsh diagram.

When the rises above the , there is more competition for resources, predators find prey more easily and disease spreads faster.
Death rate rises and falls, so the decreases.
When the falls below the , competition, predation and disease are .
rises and death rate falls, so the increases again.

Predators and their prey regulate each other in a similar way. The graph below shows the kind of cycles often seen when a predator depends mainly on one prey .

Line graph of population size in thousands against time from 0 to 40 years. A solid green prey line oscillates between 20 and 100 with peaks at about 2.5, 12.5, 22.5 and 32.5 years. A dashed red predator line oscillates between 15 and 45 with peaks at 5, 15, 25 and 35 years, each about 2.5 years after a prey peak. — Regular cycles in the numbers of a predator and its prey. Original BioWelsh graph.

When prey are plentiful, more predators survive and breed, so predator numbers rise.
More predators eat more prey, so prey numbers fall.
With less food, fewer predators survive, so predator numbers fall.
With fewer predators, prey numbers recover and the cycle repeats. The predator peak comes after the prey peak because predators take time to breed.

Lesson 3 of 19

How does competition affect populations?

Competition happens when organisms need the same resource and there is not enough for all of them. There are two types.

The two types of competition
Type	Between	Example	Usual effect
	Individuals of the same	Adult barnacles of one competing for space on a rock	Regulates size around the ; the best-adapted individuals survive
	Individuals of different	and pondweed competing for light and	One may reduce the other's or exclude it from a

is usually more intense, because members of the same need exactly the same resources. It becomes stronger as increases, which is why it helps to keep numbers close to the .

Two of the single-celled organism Paramecium both feed on bacteria. The first graph below shows them grown in separate flasks; the second shows them grown together in one flask.

Line graph of population density in cells per cubic centimetre against time from 0 to 18 days. A solid blue line for P. aurelia rises in an S-shape and levels off at about 190. A dashed red line for P. caudatum rises more slowly and levels off at about 140. — Growth of two *Paramecium* species in separate cultures. Original BioWelsh graph.

Grown separately, both show normal growth curves and level off at their own : about 190 cells cm⁻³ for P. aurelia and 140 cells cm⁻³ for P. caudatum.
Grown together, P. aurelia still grows well and levels off at about 170 cells cm⁻³.
P. caudatum rises for 6 days and then declines towards zero.
Both compete for the same food, and P. aurelia is the better competitor, so it outcompetes P. caudatum. This is .

Competition also affects where live. On rocky shores in Wales, two barnacles, Chthamalus stellatus and Semibalanus balanoides, grow on the rocks. Semibalanus grows faster and outcompetes Chthamalus lower on the shore, but it cannot survive the long periods of drying out near the top. When Semibalanus is removed from the lower shore, Chthamalus spreads down into the space, which shows that normally keeps it out.

Lesson 4 of 19

How do density-dependent and density-independent factors regulate populations?

is the number of individuals per unit area or volume, for example 50 limpets per m² or 5 000 tadpoles per m³. The factors that limit a can be sorted by whether their effect depends on this .

Density-dependent and density-independent factors
Feature	Density-dependent factors	Density-independent factors
Definition	Their effect increases as increases	They affect the same proportion of the whatever its
Usually
Examples	Competition for food or space, predation, parasitism, disease, build-up of toxic waste	Flood, fire, drought, a very cold winter, a storm, a sudden change in pH
Role	Regulate the by , keeping it close to the	Can cause sudden crashes, but do not regulate numbers

The graph below compares the two kinds of factor.

Line graph of proportion of population killed as a percentage against population density from 0 to 100 individuals per square metre. A solid blue line labelled density-dependent factor rises steadily from 4% to 52%. A dashed red line labelled density-independent factor stays flat at 20%. — How density-dependent and density-independent factors affect the proportion of a population killed. Original BioWelsh graph.

The solid line rises: at higher , a greater percentage of the is killed. This is a , such as disease, which spreads more easily when individuals are close together.
The dashed line is flat: the same 20% is killed at every . This is a , such as a severe frost.

Here is a real type of investigation. Tadpoles were added to two identical ponds, one at high and one at low . The number in the high- pond fell faster, and tadpoles there grew more slowly. This is evidence for a , such as competition for food, predation, disease or a build-up of waste.

Lesson 5 of 19

How do ecologists sample organisms in a habitat?

It is almost never possible to count every organism in a . Instead ecologists take samples and use them to estimate the whole. is how many individuals of a there are; is where in the they are found.

A sample must be representative, which it must not be biased. If you choose where to place a yourself, you may pick the most interesting spot. avoids this.

The diagram below shows how are placed at random in a study area.

A green square area divided into a 10 by 10 grid, with a tape measure along the bottom labelled x coordinate in metres and another up the left side labelled y coordinate in metres, both numbered 0 to 10. Six small white quadrats are placed at coordinates labelled (1, 2), (2, 7), (4, 5), (6, 3), (7, 6) and (8, 8). Text on the right lists five steps for placing quadrats and states that random coordinates avoid bias, so the sample represents the whole area. — Placing quadrats at random coordinates in a study area. Original BioWelsh diagram.

Two tape measures are laid at right angles along two edges of the area, making a grid of coordinates.
A random number generator or table gives a pair of coordinates, such as 6 and 3.
A is placed at that point and the organisms inside it are recorded.
This is repeated for many , and a is calculated.

A is a square frame that marks out a known area, often 0.25 m² or 1 m². The photograph below shows a being used to sample seaweed on a rocky shore.

A square frame made of grey plastic tubes lies on a rock surface covered almost completely by a bright green carpet of short seaweed. Paler bare rock is visible at the top right edge. — A quadrat used to sample green seaweed on rocks on the Basque coast of France. Photo: Marie-Noelle de Casamajor, Ifremer, CC BY 4.0. Resized.

The frame is made of plastic tubes and marks out a area of rock.
Everything inside the frame is sampled; organisms partly inside are counted using an agreed rule.
Green seaweed covers most of the area, so it would be recorded as rather than counted.

Ways of measuring abundance in quadrats
Measure	How it is found	Best for
	Count the individuals in each and divide by the area (number per m²)	Large, separate organisms, such as limpets or dandelions
	Estimate the percentage of the covered, for example by counting how many of the 100 small squares in a gridded are more than half covered	Plants and seaweeds that are hard to count individually, such as grasses
	Record whether the is present or absent in each , then find the percentage of that contain it	Quick surveys; it says nothing about how many there are
scale	Give each a category, such as abundant, common, frequent, occasional or rare (ACFOR)	Very quick surveys, but it is subjective

More give a more representative result. A good way to decide how many is to calculate a running as each is added; when the stops changing much, enough have been used.

only work for organisms that do not move. For mobile animals, such as woodlice, beetles or frogs, ecologists use .

The diagram below shows the steps and the calculation, called the Lincoln index.

Four boxes in a row joined by arrows: capture a sample and count them, n1 = 60; mark each one harmlessly, for example a paint dot; release them and wait for mixing, for example 24 hours; capture a second sample, n2 = 50 and m = 12. A box below reads estimated population = (n1 × n2) ÷ m = (60 × 50) ÷ 12 = 250. — The steps of mark, release and recapture, with a worked estimate. Original BioWelsh diagram.

Capture a sample of animals, count them (n₁) and mark each one in a way that does not harm it.
Release them where they were caught and wait long enough for them to mix back into the .
Capture a second sample, count all of them (n₂) and count how many are marked (m).
Estimated = (n₁ × n₂) ÷ m. The proportion of marked animals in the second sample is assumed to equal the proportion in the whole .

The marks do not affect survival: for example, paint does not make beetles easier for birds to see.
The marks are not lost between the two samples.
The marked animals mix completely with the rest of the .
There are no births, deaths, or between the two samples.

Lesson 6 of 19

How do transects show the distribution of organisms?

is best for comparing two areas that are each fairly uniform. But many habitats change gradually from one place to another: down a rocky shore, inland from the sea across sand dunes, or from a woodland edge into the shade. A gradual change like this in an is called an .

A is a line laid across a along an , used to sample how the of organisms changes along it. This is systematic sampling rather than .

Types of transect
Type	Method	Advantage and limitation
Line	Record every organism that touches the tape at set intervals	Quick, but only samples a very narrow strip
Belt	Place side by side along the whole tape	Very detailed, but slow over long distances
Interrupted belt	Place a at regular intervals, for example every 5 m	A good balance of detail and time for long gradients

The diagram below shows a laid inland from the sea across a sand dune system.

Side view of sand dunes rising from the sea on the left. A red tape follows the ground surface from the strandline at 0 metres over a tall yellow dune at 15 metres and a lower grey dune at 35 metres to 45 metres. Square quadrats sit on the tape every 5 metres. Labels read quadrat placed at every 5 m, interrupted belt transect, and tape laid along the environmental gradient; for a line transect, record species touching it. — An interrupted belt transect laid inland across a sand dune system. Original BioWelsh diagram.

The tape starts at the strandline, where debris is left by the highest tides, and runs inland.
It follows the : moving inland, there is more shelter from wind and salt spray, and the sand holds more water and .
A is placed at every 5 m mark, making this an interrupted belt .

Results from a are often shown as a . The below shows of four plant along the sand dune .

Kite diagram with distance from the strandline from 0 to 45 metres. Four rows are labelled with species names. Sand couch grass is widest at 10 metres and absent beyond 15 metres. Marram grass is present from 5 to 30 metres and widest at 15 to 20 metres. Red fescue is present from 20 to 45 metres and widest at 30 metres. Bird's-foot trefoil is present from 30 to 45 metres. A scale bar shows the width equal to 50% cover. — Kite diagram of percentage cover of four plant species along a sand dune transect. Original BioWelsh diagram.

Each has its own horizontal line; the width of the shape above and below the line shows its at each distance.
The scale bar shows how wide 50% cover is. At 15 m, Ammophila arenaria (marram grass) is 70% cover, so its kite is widest there.
Elymus farctus (sand couch grass) is found only in the first 15 m, nearest the sea.
Festuca rubra (red fescue) and Lotus corniculatus (bird's-foot trefoil) are found only further inland, where the dunes are more stable.

A is more useful if factors are measured at each sampling point, for example soil moisture, pH, light intensity, wind speed or height above low tide. Correlations between these and the of each suggest why live where they do.

Lesson 7 of 19

Specified practical: investigating the distribution of seaweeds on a rocky shore

A rocky shore is one of the best places to see how an affects . Near the top, organisms are covered by the sea for only a short time each day and are exposed to air, sun, wind and rain for many hours. Near the bottom, they are under water for most of the day.

Brown seaweeds called wracks are the most obvious organisms on sheltered Welsh shores. The photograph below shows knotted wrack exposed at low tide on the shore of the Menai Strait.

A thick mat of long, olive-brown, strap-like seaweed strands draped over a rock, with many small oval swellings along each strand. — Knotted wrack, *Ascophyllum nodosum*, exposed at low tide on the shore of the Menai Strait, Anglesey. Photo: Velela, Public domain.

The seaweed hangs in long, olive-brown strands over the rocks.
The oval swellings are air bladders, which float the seaweed up towards the light when the tide covers it.
Out of the water, the seaweed is exposed to drying out, which is why each has a limited on the shore.

Different wracks form bands, or , at different heights. high on the shore, such as Pelvetia canaliculata (channelled wrack), tolerate drying out. low on the shore, such as Fucus serratus (serrated wrack), cannot survive long out of water, but grow fast and outcompete other seaweeds below the mid shore.

Specified practical

Investigating the abundance and distribution of seaweeds on a rocky shore

Unit 3, section 5, specified practical 1 (BioWelsh entry number)

Aim To investigate how the and of seaweed change with height down a rocky shore, using an interrupted belt .

Investigating seaweed distribution on a rocky shore at a glance. Original BioWelsh diagram.

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Equipment

Ranging poles
Ranging poles2 poles, 2 m, marked in 10 cm bandsMeasure the drop in height between sampling points.
Tape measure
Tape measure50 mMarks out the line down the shore.
Spirit level
Spirit levelKeeps the tape horizontal between the ranging poles.
Gridded quadrat
Gridded 0.5 m × 0.5 mMarks a area; 100 squares help estimate .
Seaweed identification guide
Seaweed identification guideIdentifies each correctly.
Clipboard and recording sheet
Clipboard and recording sheetRecords cover and heights at each .
Boots with good grip
Boots with good gripReduce the risk of slipping on wet seaweed.

Safety

Wet, seaweed-covered rocksRiskSlipping and falling when walking down the shore.ControlWear boots with good grip and do not run.
Incoming tideRiskBeing cut off when working on the lower shore.ControlCheck tide times, work down the shore after the tide and keep watching the water.
Sharp barnacles and broken shellsRiskCuts to hands when placing .ControlWear gloves and cover cuts.

Variables

Independent variableHeight on the shore (distance along the transect)

every 5 m; height above lowest tide measured with ranging poles.

Height determines how long each point is exposed to air.

Dependent variableAbundance of each seaweed species

in each , using a gridded .

is used because seaweeds grow as tangled masses.

Controlled variablesQuadrat size, Interval between quadrats, Rule for estimating cover, Aspect and exposure to waves, Time of year and tide

size

The same 0.5 m × 0.5 m gridded throughout.

A larger would include more seaweed.

Interval between

Every 5 m along the tape.

Unequal gaps would give a misleading pattern.

Rule for estimating cover

Count squares more than half covered, by the same person.

Different rules give different estimates.

Aspect and exposure to waves

Repeat on the same stretch of shore facing the same way.

Wave exposure also affects which seaweeds grow.

Time of year and tide

Carry out all on the same day at low tide.

Seaweed growth changes with season.

Method, step by step

1)
Check the tide tables and plan the survey for the day with the lowest tide, starting about two hours before low tide.
Why?
More of the shore is uncovered for longer, so the can reach the lower shore safely.
2)
Lay the tape measure in a straight line from the top of the shore down to the water's edge, at right angles to the sea.
Why?
The follows the of time exposed to air.
3)
Place the gridded at 0 m with its corner on the tape, always on the same side of the tape.
Why?
The same position at each point avoids choosing with more seaweed.
4)
Identify each seaweed in the and estimate its by counting the small squares that are more than half covered.
Why?
Seaweeds cannot be counted as individuals, so measures .
5)
Stand a ranging pole at 0 m and another at 5 m, hold the tape horizontally between them with a spirit level, and read the drop in height on the lower pole.
Why?
Adding up the drops gives the height of each , which explains the pattern of .
Measuring the drop in height between two points on a shore with ranging poles. Original BioWelsh diagram.
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6)
Move down the shore and repeat the and height measurements every 5 m to the water's edge.
Why?
Regular intervals give an interrupted belt that samples the whole gradient in the time available.
7)
Repeat the at two more places on the same shore, at least 10 m to each side.
Why?
Repeats show whether the pattern is typical of the shore, and allow a at each distance.
8)
Calculate the of each at each distance, and plot a and a shore profile.
Why?
A shows how the of each changes along the .

Results and analysis

Example data

of five seaweed and height above the lowest tide at each along the

Distance / m	Height / m	Channelled wrack / % cover	wrack / % cover	Bladder wrack / % cover	Serrated wrack / % cover	Oarweed / % cover
0	5.8	30	0	0	0	0
5	5.1	55	15	0	0	0
10	4.3	10	60	0	0	0
15	3.4	0	25	40	0	0
20	2.7	0	5	70	0	0
25	2.0	0	0	35	30	0
30	1.2	0	0	5	65	0
35	0.5	0	0	0	40	25
40	0.1	0	0	0	0	80

Kite diagram with distance from the top of the shore from 0 to 40 metres. Channelled wrack is present from 0 to 10 metres and widest at 5 metres. Spiral wrack is present from 5 to 20 metres and widest at 10 metres. Bladder wrack is present from 15 to 30 metres and widest at 20 metres. Serrated wrack is present from 25 to 35 metres and widest at 30 metres, narrowing to nothing at 40 metres. Oarweed is present from 35 metres and widest at 40 metres. A scale bar shows the width equal to 50% cover. — Kite diagram of percentage cover of five seaweed species along a rocky shore transect. Original BioWelsh diagram.

Profile of the rocky shore along the transect. Original BioWelsh graph.

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What the results show

Each has a . Pelvetia canaliculata has its highest cover (55%) at 5 m, 5.1 m above the lowest tide, and Laminaria digitata has its highest cover (80%) at 40 m, only 0.1 m above it. The three Fucus form bands in between.

The match the . The top of the shore is uncovered by the sea for many hours each day, so only that tolerate drying out, such as P. canaliculata, can survive there.

Lower down, that lose water quickly can survive, and they grow faster, so they outcompete the upper shore . for space and light helps set the lower limit of each zone.

The overlap, so no single height separates two exactly. The shows this overlap clearly.

Worked calculations

Height of the at 10 m = 5.8 − (0.7 + 0.8) = 4.3 m above lowest tide

cover of F. vesiculosus at 20 m from three of 72, 65 and 73% = (72 + 65 + 73) ÷ 3 = 70%

Evaluation

Percentage cover is estimated by eye.

Effect on resultsDifferent people give different values, so results are less reproducible.

ImprovementUse one person throughout, or use a point and count hits.

Seaweeds lie on top of each other.

Effect on resultsTotal cover in a can exceed 100%, and lower layers may be missed.

ImprovementLift the top layer carefully to record each layer.

Quadrats are only every 5 m.

Effect on resultsNarrow or sharp boundaries may be missed.

ImprovementUse a belt across boundaries.

Only three transects on one shore.

Effect on resultsThe pattern may not be typical of shores with different wave exposure.

ImprovementRepeat on a sheltered and an exposed shore.

Going further

Random sampling or a transect?

Use a when an changes gradually across the , as it does down a shore, so that sampling follows the change. Use random with coordinates from a random number generator when comparing two areas that are each fairly uniform, such as grazed and ungrazed grassland.

Other ways to carry out this practical

Sand dunes: run a inland from the strandline and record of plants with soil moisture and pH.
Woodland edge: run a from open ground into the wood and measure light intensity with a light meter.
Trampled path: run a across a footpath to see how trampling affects plant .
Rivers: kick sampling, where stones inside a are disturbed for a set time and animals are caught in a net placed downstream.
Mobile animals: estimate size by .

An abundance scale

Abundant: more than 50% cover.
Common: 20 to 50% cover.
Frequent: 5 to 20% cover.
Occasional: 1 to 5% cover.
Rare: less than 1% cover.

Model answer: describe the method

Describe how you would investigate the of seaweed down a rocky shore.

Original practice response · 6 indicative marks

Visit on the day of the lowest tide, having checked the tide times, and wear boots with good grip.
Lay a tape measure from the top of the shore to the sea, along the .
Place a of size at regular intervals along the tape, such as every 5 m.
In each , identify the seaweeds with a key and estimate the of each .
Measure the height of each above low tide using ranging poles and a spirit level.
Repeat the at other points on the same shore, calculate and plot a .

What the answer needs to cover

along the
of size at regular intervals
or number of each recorded
Height or exposure measured at each point
Repeats and
Safety or timing to the tide

Exam practice

Explain why a transect was used instead of placing quadrats at random on the shore.Show answer

A follows the down the shore. conditions, such as time exposed to air, change with height, so the of seaweeds changes and the samples this change systematically.

Line along an
changes down the shore

Suggest why the students carried out the survey on the day with the lowest tide.Show answer

The lowest tide uncovers the greatest length of shore for the longest time, so the can reach the lower shore and there is more time to sample.

Greatest length of shore uncovered
More time available for sampling

State two precautions that should be taken to reduce harm to the organisms or their habitat.Show answer

Record organisms where they are found rather than removing them, and replace any rocks or seaweed that are moved; avoid stepping on organisms where possible.

Do not remove organisms
Replace rocks and seaweed

Lesson 8 of 19

What are ecosystems, habitats and communities?

Ecology uses a small set of terms that build on each other. You need to use them precisely.

Key terms in ecology
Term	Meaning	Example on a rocky shore
	All the organisms of one in one place at one time	All the limpets on the shore
	All the of different living and interacting in the same place at the same time	The limpets, barnacles, seaweeds, crabs and fish on the shore
	The place where an organism lives	A rock pool, or the underside of a boulder
	A together with the environment it interacts with	The whole shore: its organisms, rocks, water, air and sunlight
	A living factor that affects organisms	Predation by dog whelks; competition for space
	A non-living factor that affects organisms	Wave action, temperature, salinity, time exposed to air

Each also has a : its role in the , including what it eats, what eats it and the conditions it needs. As a becomes more complex, it offers more niches, so it can support more .

Ecosystems range in size from very small to very large
Size	Example
Very small	The water in a hollow tree stump; the gut of a termite
Small	A rock pool; a rotting log
	A pond; an oak woodland; a sand dune system
Very large	A tropical rainforest; an ocean; the whole biosphere

Almost every depends on the Sun. , such as plants and algae, use light energy in to make organic molecules. All other organisms get their energy, directly or indirectly, from these molecules. Energy flows through an and is eventually lost as heat, so it must be constantly supplied by sunlight. Nutrients such as carbon and nitrogen are not lost: they cycle.

Lesson 9 of 19

What are gross and net primary production?

Primary production is the making of new organic material by . It is where energy first enters a food chain, so it sets a limit on everything else in the .

Gross and net primary production
Term	Meaning
()	The rate at which convert light energy into chemical energy in organic molecules by
(R)	The part of that use in their own , released as heat
()	The chemical energy left in the ' after ; it is available to the next

The three are by one equation: = − R. Production is a rate, so it is given per unit area per unit time, for example kJ m⁻² year⁻¹ for energy or g m⁻² year⁻¹ for dry .

Only a small fraction of the light reaching a plant ends up in new . The flow diagram below shows approximate values for a crop leaf in good growing conditions.

Vertical flow of four boxes: light energy reaching the leaf 100%, absorbed by chlorophyll 40%, gross primary production 4%, net primary production 3%. Grey arrows lead to the right from each of the first three boxes: not absorbed 60%, made up of wrong wavelength 45%, reflected 10% and transmitted through the leaf 5%; lost as heat and in evaporating water 36%; released in the plant's own respiration 1%. Text reads NPP = GPP minus respiration. — What happens to the light energy reaching a crop leaf. Original BioWelsh diagram.

About 60% of the light is not absorbed at all: it is the wrong , is reflected, or passes straight through the leaf.
Most of the absorbed light is lost as heat or used to evaporate water.
Only about 4% of the light is converted to chemical energy in : this is the .
About a quarter of the is used in the plant's own , leaving about 3% as .

Light may not reach the : it can miss the leaves, be reflected or be transmitted through them.
Chlorophyll absorbs only certain , mainly red and blue light.
Other factors, such as carbon dioxide concentration, temperature, water and mineral ions, may limit the rate of .
Some of the energy is used in the plant's own .

can be measured by the harvest method. Identical plots of vegetation are chosen. One is harvested at the start, dried and weighed. Another is harvested after a set time. The increase in per m² per day is the . A third plot can be covered with black plastic so that no happens: its loss in shows the rate of .

Approximate mean net primary production of some ecosystems
Ecosystem	NPP / g m⁻² year⁻¹	Main reasons
Tropical rainforest	2 200	Warm all year, high rainfall and light intensity, a long growing season and many layers of leaves
Temperate deciduous forest	1 200	A shorter growing season; trees lose their leaves in winter and it is colder
Farmland	650	A growing season of only a few months; crops cover the ground for part of the year
Tundra	140	Very cold, with a short growing season
Desert	90	Too little water; few plants, so little light is intercepted

Lesson 10 of 19

How efficiently is energy transferred along food chains?

Each stage in a food chain is a . are the first , primary consumers (herbivores) the second, secondary consumers the third and tertiary consumers the fourth. Most animals feed at more than one level, so real feeding relationships form food webs.

When an animal eats, only part of the energy in its food becomes new . The diagram below shows typical values for a herbivore.

Vertical flow of three boxes: energy in plant food eaten 100%, energy absorbed from the gut 40%, secondary production (new biomass) 10%. A brown arrow from the first box leads to egested in faeces 60%, indigestible cellulose and lignin. A red arrow from the second box leads to released in respiration and lost as heat 30%. Text beside the last box reads available to the next trophic level, only part of it is eaten. — What happens to the energy in the food eaten by a herbivore. Original BioWelsh diagram.

Herbivores cannot digest much of their food, such as cellulose and lignin, so a large part is egested in faeces.
Of the energy absorbed, most is released in to power movement and other processes, and is lost as heat.
Some is lost in , for example as in urine.
Only about 10% of the energy eaten becomes new , called , which is available to the next .

Why energy is lost between trophic levels
Loss	Explanation
Not all of the organism is eaten	Roots, woody stems, bones and fur are often left
Not all of what is eaten is digested	Cellulose, lignin, bone and hair pass out in faeces
	Some energy leaves in waste such as
	Energy released is used for movement and body processes and is lost as heat; mammals and birds lose extra heat keeping warm

The diagram below shows energy flow through a whole grassland food chain, including . All values are in kJ m⁻² year⁻¹.

Four boxes in a row joined by arrows: producers with GPP 30 000 and NPP 20 000, arrow 2 400 to primary consumers 2 400, arrow 250 to secondary consumers 250, arrow 45 to tertiary consumers 45. Red arrows up from each box to a band labelled released in respiration and lost as heat carry 10 000, 1 300, 130 and 30. Brown arrows down to a band labelled dead remains, faeces and urine, used by detritivores and decomposers, carry 17 600, 850, 75 and 15. A red arrow below reads decomposers respire, 18 540 lost as heat. — Energy flow through a grassland food chain, in kJ m⁻² year⁻¹. Original BioWelsh diagram.

fix 30 000 by (). They respire 10 000, leaving 20 000 as .
Primary consumers take in 2 400. Of this, 1 300 is respired, 850 goes to dead remains and faeces, and 250 passes to secondary consumers.
At every level, arrows up show energy lost as heat in , and arrows down show energy passing to and .
The eventually respire all of the energy they receive, so it too is lost as heat.

Because roughly 90% of the energy is lost at each step, there is too little energy left to support a fifth or sixth . This is why food chains rarely have more than four or five levels, and why top predators are always few in number.

Lesson 11 of 19

What do pyramids of numbers, biomass and energy show?

Ecological pyramids show one feature of each as a horizontal bar, with at the bottom. There are three kinds.

The diagram below shows all three pyramids for the same grassland food chain: grass, grasshoppers, frogs and grass snakes.

Three stepped pyramids side by side for grass, grasshoppers, frogs and grass snakes. Numbers per hectare: 2 000 000, 30 000, 90 and 2. Biomass in grams per square metre: 5 000, 120, 8 and 1. Energy in kilojoules per square metre per year: 21 000, 2 000, 240 and 40. Each pyramid narrows towards the top. A note says bar lengths are not drawn to scale. — Pyramids of numbers, biomass and energy for one grassland food chain. Original BioWelsh diagram.

The pyramid of numbers counts the individuals at each level in a given area.
The shows the of organisms per m² at each level at one time.
The shows the energy flowing through each level per m² per year.
In this food chain all three have the typical pyramid shape, narrowing towards the top.

Comparing the three types of pyramid
Pyramid	Advantage	Disadvantage
Numbers	Easy to count	Ignores size: one oak tree counts the same as one caterpillar, so it can be inverted
	Takes account of size	Measured at one moment, so it can be inverted when reproduce very quickly; organisms must be killed and dried
Energy	Shows how efficiently energy is transferred; never inverted	Very difficult to measure, and needs data collected over a whole year

Pyramids of numbers and are sometimes inverted, as shown below.

Left: a pyramid of numbers for one oak tree with a narrow bottom bar for 1 oak tree, a very wide bar for 40 000 caterpillars, a medium bar for 25 blue tits and a narrow top bar for 1 sparrowhawk. Right: a pyramid of biomass for a sea in winter with a narrow bottom bar for phytoplankton, 5 grams per square metre, and a wider bar above for zooplankton, 18 grams per square metre. — An inverted pyramid of numbers and an inverted pyramid of biomass. Original BioWelsh diagram.

One large oak tree supports tens of thousands of caterpillars, so the bar of the pyramid of numbers is narrower than the bar above it.
In the sea in winter, the of phytoplankton at any moment can be smaller than the zooplankton feeding on them.
This is possible because phytoplankton reproduce very quickly: they are eaten almost as fast as they are made, so a snapshot underestimates how much they produce.

Lesson 12 of 19

How does succession change bare rock into woodland?

are not . Over time, the living in an area changes, as each group of alters the environment in ways that let other move in. This gradual, directional change in a over time is called .

Primary and secondary succession
Feature	Primary succession	Secondary succession
Starts on	Bare ground with no soil: bare rock, a new volcanic island, sand, or rock left by a retreating glacier	Ground where soil is already present, after a has been removed: a forest fire, clear-felling, or an abandoned field
Speed	Very slow, often hundreds of years	Faster, because soil, seeds, spores, roots and nutrients are already there

The first organisms to colonise bare rock are , such as lichens. The photograph below shows crustose lichens growing directly on a rock surface.

Close-up of a rock surface covered in flat, crust-like patches of orange, grey-blue and white lichen, dotted with many small black discs. — Crustose lichens growing on a bare rock surface in Alaska. Photo: Bering Land Bridge National Preserve, US National Park Service, CC BY 2.0. Resized.

The lichens form flat, crust-like patches of different colours that are firmly attached to the rock.
They need no soil: they get water from rain and make their own food by .
The small dark discs are reproductive structures that release spores, which are carried by the wind to new rock.

tolerate harsh conditions: extremes of temperature, drying out, strong light and no soil.
Their spores or seeds are small and spread easily by wind.
Lichens release acids that slowly weather the rock surface.
When die, break them down to form . Mixed with rock particles, this makes a thin, simple soil.

from bare rock to woodland in lowland Britain

1) Bare rock
2) : lichens
3) Mosses
4) Grasses, ferns and small flowering plants
5) Shrubs such as bramble and hawthorn
6) Fast-growing trees such as birch
7) : oak woodland

Each in the sequence is a , and the whole sequence is called a . At each stage, the organisms change the conditions: soil becomes deeper, holds more water and contains more and mineral ions, and taller plants create shade and shelter. These changes make conditions more suitable for new , which then outcompete the existing ones, for example by shading them out.

Eventually a stable develops that stays much the same over time, as long as the conditions do not change. This is the . In most of lowland Britain it is deciduous woodland, such as oak; on thin chalk soils it may be beech. Where soil stays waterlogged, as in a peat bog, the climax may be dominated by Sphagnum moss instead of trees.

The graph below shows how some features of an change during .

Line graph of value as a percentage of the maximum against time since bare rock was exposed from 0 to 200 years. A solid brown line for soil depth rises steadily, curving to about 94% at 200 years. A dashed green line for biomass stays low until about 50 years, rises steeply between 75 and 150 years and levels off near 98%. A dotted blue line for number of species rises steeply to a peak of about 87% at 115 years, then falls to about 73%. — How soil depth, biomass and number of species change during primary succession. Original BioWelsh graph.

Soil depth increases steadily as more organic matter is added and rock is weathered.
rises slowly at first, then rapidly as shrubs and trees grow, and levels off at the climax.
The number of rises as conditions improve and more habitats and food sources appear.
number falls slightly near the climax, as a few trees shade out other plants.

Lesson 13 of 19

Why is the breakdown of dead organic matter important?

Living organisms constantly take in elements such as carbon, nitrogen and phosphorus to build their molecules. The Earth has a amount of each, so they must be returned to the environment and reused. Without decay, these elements would stay locked up in dead bodies and waste, and would run out of the mineral ions they need.

The organisms that break down dead material
Group	Examples	How they feed
	Earthworms, woodlice, millipedes	Eat small pieces of dead material (detritus) and digest it inside their gut; breaking it up increases the surface area for
()	Many bacteria and fungi	Secrete onto dead material and absorb the soluble products: this is extracellular digestion

use the products of digestion as respiratory , releasing carbon dioxide into the air or water. They also release mineral ions, such as ammonium and phosphate ions, into the soil, where plants can absorb them. Decay is fastest when it is warm, moist and there is plenty of oxygen, because use and most respire aerobically.

Energy flow compared with nutrient cycling
Feature	Energy	Nutrients such as carbon and nitrogen
Source	Sunlight	Already present in the
Route	Flows in one direction along food chains	Cycle between organisms and the environment
Fate	Lost as heat and must be constantly replaced	Returned to the environment by and reused

Lesson 14 of 19

How is carbon cycled in an ecosystem?

Carbon is part of every organic molecule: carbohydrates, lipids, proteins and nucleic acids. It moves between the atmosphere, the oceans, living organisms and rocks in the carbon cycle.

The diagram below shows the main stores of carbon and the processes that move carbon between them.

Flow diagram of six boxes: carbon dioxide in the atmosphere at the top; carbon compounds in producers and in consumers in the middle; carbon compounds in dead organisms, faeces and urine below; fossil fuels and carbonates in shells, chalk and limestone at the bottom. Arrows show photosynthesis from the atmosphere to producers, respiration from producers and consumers to the atmosphere, feeding from producers to consumers, death from producers and death and egestion from consumers to dead matter, decomposers respire from dead matter to the atmosphere, fossilisation from dead matter to fossil fuels, combustion from fossil fuels to the atmosphere, shells and skeletons from consumers to carbonates, and weathering of rocks from carbonates to the atmosphere. — The carbon cycle. Original BioWelsh diagram.

removes carbon dioxide from the atmosphere, and use it to make organic molecules. (The first step, fixing carbon dioxide to , is covered in the Photosynthesis lesson on the light-independent stage.)
Carbon passes to consumers when they eat, and to dead matter when organisms die, egest faeces or excrete.
by , consumers and returns carbon dioxide to the atmosphere.
Where dead matter is buried in waterlogged, acidic or oxygen-poor conditions, cannot work. Over millions of years, the organic matter forms peat and then .
Combustion of and wood releases their carbon as carbon dioxide.
Carbon dioxide dissolves in the oceans as hydrogencarbonate ions. Marine organisms use carbonate to build shells, which can form chalk and limestone. Weathering of these rocks and volcanic activity return carbon dioxide slowly.

Before large-scale human activity, the amount of carbon dioxide removed by was roughly balanced by the amount released by and decomposition, so the concentration in the atmosphere stayed fairly constant.

Lesson 15 of 19

How do human activities affect the carbon cycle and climate?

Humans have upset the balance of the carbon cycle, mainly in two ways: burning , and . As a result, the concentration of carbon dioxide in the atmosphere is rising.

The graph below shows measurements of atmospheric carbon dioxide made at the Mauna Loa observatory in Hawaii, far from local sources of pollution.

Line graph with crosses of carbon dioxide concentration in parts per million against year from 1960 to 2030. Values rise from 317 in 1960 through 339 in 1980, 370 in 2000 and 414 in 2020 to 425 in 2024, with the line becoming steeper over time. — Mean atmospheric carbon dioxide concentration at Mauna Loa, Hawaii, 1960 to 2024, rounded from NOAA records. Original BioWelsh graph.

In 1960 the concentration was about 317 parts per million (ppm).
By 2024 it had risen to about 425 ppm, an increase of about a third.
The line gets steeper over time, so the rate of increase is itself increasing.

The photograph below shows an area of forest after the trees were cleared by clear-felling.

Black and white photograph of a hillside where almost all the trees have been cut down. Stumps, fallen branches and debris cover the ground, a few thin trunks still stand, and forested hills are visible in the distance. — Clear-felled forest in north Georgia, USA, photographed in the early 1900s. Photo: Chattahoochee-Oconee National Forests, US Forest Service, CC BY 2.0. Resized.

Almost all the trees have been cut down, leaving stumps, dead branches and a few scattered standing trunks.
Branches and other debris cover the ground, where they will decompose or may be burned.
Forest remains only on the distant hills.

How deforestation affects atmospheric carbon dioxide
Effect	Reason
Less carbon dioxide removed	Fewer trees carry out
More carbon dioxide released	Trees and waste wood are burned, and the dead roots, leaves and branches are decomposed by that respire
Carbon store lost	Carbon locked up in wood for hundreds of years is released quickly
A small opposite effect	With fewer living trees and animals, there is less in the forest

Carbon dioxide and methane are . The diagram below shows the .

Diagram with the Sun at top left, a grey band across the middle labelled atmosphere with greenhouse gases, carbon dioxide, methane and water vapour, and the curved green Earth's surface at the bottom. A thick yellow arrow labelled short-wavelength radiation passes through goes from the Sun to the surface. A red arrow labelled surface emits infrared rises from the surface; above the atmosphere it continues as a dashed arrow labelled some infrared radiation escapes to space. Another red arrow comes down from the atmosphere to the surface, labelled infrared radiation absorbed by greenhouse gases and re-radiated back to Earth, warming it. — The greenhouse effect. Original BioWelsh diagram.

Short- radiation from the Sun passes through the atmosphere and warms the Earth's surface.
The warm surface emits longer- infrared radiation.
Some infrared escapes to space, but absorb much of it and re-radiate it, including back towards the Earth.
This natural effect keeps the Earth warm enough for life. Increasing the concentration of traps more heat, causing .

Methane is released by cattle and sheep, from rice fields, from landfill sites and when wetlands are drained. Molecule for molecule it traps far more heat than carbon dioxide.

changes the climate: average temperatures rise, rainfall patterns change, sea levels rise as ice melts and water expands, and the oceans become more acidic as they absorb carbon dioxide. Extreme weather events such as droughts and floods become more frequent.

The of many shifts towards the poles, or to higher altitudes, as their warms.
that cannot move quickly enough, or that already live at the top of mountains or in polar regions, have nowhere to go, so their fall and they may become .
Timings can fall out of step: for example, if caterpillars emerge earlier in a warm spring, birds may hatch their chicks after the peak of food.
Coral reefs lose their algae and die in warmer, more acidic water, which removes the of many other .
from warmer regions, including pests and disease-carrying insects, can spread into new areas.

Lesson 16 of 19

What is the role of bacteria in the nitrogen cycle?

Nitrogen makes up about 78% of the air, but plants and animals cannot use nitrogen gas (N₂), because its two atoms are held by a very strong triple bond. Nitrogen must first be converted into ions, such as ammonium or . Bacteria carry out almost every step of the nitrogen cycle.

Living things need nitrogen to make amino acids, and so proteins, including . They also need it for the bases of nucleotides, and so DNA, RNA and , and plants need it for chlorophyll. Plants absorb from the soil by into root hair cells. (Why a lack of stunts growth is covered in the Photosynthesis lesson on why plants need nitrogen and magnesium.)

The diagram below shows the nitrogen cycle and the bacteria involved.

Flow diagram of six boxes: nitrogen gas in the air at the top; proteins and nucleic acids in plants on the left, and in animals below; ammonium ions, nitrite ions and nitrate ions in soil. A blue arrow from nitrogen gas to ammonium is labelled nitrogen fixation by Rhizobium in root nodules and by free-living Azotobacter. Feeding links plants to animals. Brown arrows from plants and animals to ammonium are labelled death, urea and faeces, and ammonification by decomposers. A grey arrow from ammonium to nitrite is labelled Nitrosomonas, nitrification. A purple arrow from nitrite to nitrate is labelled Nitrobacter, nitrification. A green arrow from nitrate to plants is labelled absorbed by roots. A red arrow from nitrate to nitrogen gas is labelled denitrification by Pseudomonas in anaerobic soil. — The nitrogen cycle and the bacteria involved. Original BioWelsh diagram.

: bacteria convert nitrogen gas into ammonium ions. Rhizobium lives in of such as clover, peas and beans; Azotobacter lives free in the soil.
: when plants and animals die, or animals excrete and egest faeces, bacteria and fungi break down proteins and nucleic acids and release ammonium ions (NH₄⁺).
, step one: Nitrosomonas oxidises ammonium ions to nitrite ions (NO₂⁻).
, step two: Nitrobacter oxidises nitrite ions to (NO₃⁻), which plants absorb. Nitrifying bacteria need oxygen.
: in waterlogged, anaerobic soils, bacteria such as Pseudomonas convert back to nitrogen gas, which is lost to the air.

Bacteria in the nitrogen cycle
Bacteria	Conversion	Conditions
Rhizobium (in ); Azotobacter (free-living)	N₂ → ammonium ions (NH₄⁺)	Anaerobic within the nodule; needs a lot of
bacteria and fungi	Proteins, and nucleic acids → ammonium ions	Warm, moist
Nitrosomonas, then Nitrobacter	NH₄⁺ → NO₂⁻ → NO₃⁻	Aerobic
Pseudomonas	NO₃⁻ → N₂	Anaerobic, for example waterlogged soil

The photograph below shows the roots of a soybean plant, a , with many .

Yellowish roots arranged on a dark background, with many small rounded swellings clustered along the roots. — Root nodules on the roots of a soybean plant, which contain nitrogen-fixing bacteria. Photo: USDA Agricultural Research Service, Public domain.

The long yellow strands are roots, shown against a dark background.
The small rounded swellings clustered along the roots are .
Each nodule contains millions of Rhizobium bacteria fixing nitrogen.

The simple diagram below explains how a works.

Simple section diagram. On the left, a brown circle labelled root cross-section contains a green central circle labelled xylem and phloem. On the right, a larger pink circle labelled root nodule has an inner region packed with small red rod shapes. A green arrow runs from the nodule to the root's central tissue and a yellow arrow runs back. Labels read: nitrogen gas diffuses in from air spaces in the soil; cells packed with Rhizobium, nitrogenase reduces nitrogen gas to ammonium; leghaemoglobin binds oxygen, keeping oxygen low so nitrogenase can work; a key shows amino acids from the bacteria go to the plant in the xylem and sugars go to the bacteria in the phloem. — How a root nodule fixes nitrogen. Original BioWelsh diagram.

Nitrogen gas diffuses into the nodule from air spaces in the soil.
Inside the nodule, cells are packed with Rhizobium. Their , , reduces nitrogen gas to ammonium ions, which are used to make amino acids.
is inhibited by oxygen, so the nodule contains , a pink protein that binds oxygen and keeps the concentration very low.
The plant receives amino acids through its xylem, and the bacteria receive sugars from the phloem for . Both benefit, so this is a relationship.

Lesson 17 of 19

How do farming practices affect the nitrogen cycle?

When crops are harvested, the nitrogen in them is taken away from the field instead of being returned to the soil by decay. Over time, the soil loses , and crop yields fall. Farmers use several practices to keep levels high, which is why the nitrogen cycle is so important economically.

How farming practices affect soil nitrate
Practice	Effect on the soil	Effect on the nitrogen cycle
Ploughing	Breaks up and turns the soil, letting air in	Aerobic conditions favour nitrifying bacteria and reduce , so more forms
Drainage	Removes excess water from waterlogged soil, so air fills the spaces	increases and decreases; roots also respire aerobically and can absorb ions by
Adding inorganic	Adds ammonium or other soluble ions directly	is immediately available, but excess may leach into rivers
Adding manure	Adds organic matter, which improves soil structure	release ammonium ions slowly, which are then nitrified
Growing in	Clover, peas or beans are grown and then ploughed back into the soil	Nitrogen by Rhizobium in is added to the soil, so less is needed

increases crop yield, but it is expensive to make and apply, and using more than the crop can absorb wastes money and harms the environment. Growing reduces costs. The best practice is to apply the right amount at the right time, when crops are growing and can take it up.

Waterlogged soil shows why oxygen matters. Water fills the air spaces, so the soil becomes anaerobic. bacteria convert to nitrogen gas and nitrifying bacteria cannot work, so levels fall. Roots also cannot respire aerobically, so they cannot actively absorb the few ions that remain. This is why trees cannot grow in bogs, and why insectivorous plants such as sundews, which trap and digest insects to obtain nitrogen, succeed there.

Lesson 18 of 19

What causes eutrophication and algal blooms?

and phosphate ions are very soluble. When more is applied than crops can absorb, or when heavy rain follows spreading, the ions are washed through the soil into streams, rivers and lakes. This is called . Sewage, animal slurry and waste from fish farms add more. An increase in the nutrient concentration of water is called .

The sequence of events in

1) and phosphate leach into water
2) Algae grow and reproduce rapidly: an
3) The bloom blocks light from plants below the surface
4) Submerged plants cannot photosynthesise and die; algae also die
5) Aerobic multiply and respire, using up dissolved oxygen
6) Fish and other aerobic animals die

The photograph below shows an on a farm pond.

A pond whose surface is covered by a green scum of algae with a mottled pattern, bordered by bright green grass in the foreground. — An algal bloom on a farm pond in West Virginia, USA. Photo: Eric Vance, US Environmental Protection Agency, Public domain.

A thick green scum of algae covers much of the water surface.
Light cannot pass through the scum, so plants growing under the water are shaded.
The pond is surrounded by farmland, a likely source of and phosphate.

The key point is that the fish die from lack of oxygen, not from the itself. As bacteria decompose the large amount of dead plant and algal material, their uses oxygen faster than it can dissolve from the air. The water may become anaerobic, and anaerobic bacteria then produce toxic gases such as hydrogen sulfide, and carry out .

Draining land for farming also harms habitats directly. Wetlands such as marshes, fens and bogs dry out, so the plants, insects and birds adapted to wet conditions lose their and biodiversity falls. Drained peat is decomposed, releasing carbon dioxide. The water drained from fields also carries into rivers, adding to .

Apply only when crops are growing, and not before heavy rain.
Leave strips of uncultivated land beside rivers to absorb run-off.
Use manure or , which release nitrogen more slowly.
Remove phosphate from sewage at treatment works.

Lesson 19 of 19

How do the ideas in population size and ecosystems fit together?

How the ideas in this topic connect

Zoom 100%

Zoom in to read each branch. Swipe or scroll sideways to explore.

size and
- - Change = (births + ) − (deaths + )
  - Growth curve levels off at the
  - Regulated by competition and factors
- Sampling
  - Random avoid bias; for animals
  - follow an ; kite diagrams show
- Energy flow
  - Sun to : , and = − R
  - About 10% passes between ; pyramids show this
- Change over time
  - : , seral stages,
- Nutrient cycles
  - release carbon dioxide and ammonium ions
  - Carbon cycle: human activity raises carbon dioxide and changes climate
  - Nitrogen cycle: , , ,
  - Ploughing and drainage help ; excess causes

A stable has + = death rate + .
Name the type of competition and the resource being competed for.
factors are usually and regulate the ; factors are usually .
avoids bias; a follows an .
= − R; it is the energy available to the next .
Energy flows and is lost as heat; nutrients cycle.
Pyramids of energy are never inverted.
starts with soil present, so it is faster than .
gives ammonium ions, not ; Nitrosomonas makes nitrite and Nitrobacter makes .
In , fish die because use up the oxygen.

How do populations grow and change in size?

What is carrying capacity and what limits population growth?

How does competition affect populations?

How do density-dependent and density-independent factors regulate populations?

How do ecologists sample organisms in a habitat?

How do transects show the distribution of organisms?

Specified practical: investigating the distribution of seaweeds on a rocky shore

Investigating the abundance and distribution of seaweeds on a rocky shore

Equipment

Safety

Variables

Method, step by step

Results and analysis

of five seaweed and height above the lowest tide at each along the

Kite diagram of percentage cover of five seaweed species along a rocky shore transect. Original BioWelsh diagram.

Profile of the rocky shore along the transect. Original BioWelsh graph.

What the results show

Worked calculations

Evaluation

Going further

Model answer: describe the method

Exam practice

What are ecosystems, habitats and communities?

What are gross and net primary production?

How efficiently is energy transferred along food chains?

What do pyramids of numbers, biomass and energy show?

How does succession change bare rock into woodland?

Why is the breakdown of dead organic matter important?

How is carbon cycled in an ecosystem?

How do human activities affect the carbon cycle and climate?

What is the role of bacteria in the nitrogen cycle?

How do farming practices affect the nitrogen cycle?

What causes eutrophication and algal blooms?

How do the ideas in population size and ecosystems fit together?

Worksheets

Worksheet 1: 3.5 Populations and ecosystems (147 marks)

Check your understanding.

A population of 250 red squirrels in a forest was monitored for one year. There were 60 births and 45 deaths, 12 squirrels moved into the forest and 20 moved out. What was the population size at the end of the year?

Distance / m	Height / m	Channelled wrack / % cover	wrack / % cover	Bladder wrack / % cover	Serrated wrack / % cover	Oarweed / % cover
0	5.8	30	0	0	0	0
5	5.1	55	15	0	0	0
10	4.3	10	60	0	0	0
15	3.4	0	25	40	0	0
20	2.7	0	5	70	0	0
25	2.0	0	0	35	30	0
30	1.2	0	0	5	65	0
35	0.5	0	0	0	40	25
40	0.1	0	0	0	0	80

Distance / m	Height / m	Channelled wrack / % cover	wrack / % cover	Bladder wrack / % cover	Serrated wrack / % cover	Oarweed / % cover
0	5.8	30	0	0	0	0
5	5.1	55	15	0	0	0
10	4.3	10	60	0	0	0
15	3.4	0	25	40	0	0
20	2.7	0	5	70	0	0
25	2.0	0	0	35	30	0
30	1.2	0	0	5	65	0
35	0.5	0	0	0	40	25
40	0.1	0	0	0	0	80

Population size and ecosystems

How do populations grow and change in size?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 10 marks in total. This is a revision priority, not a prediction of your exam.

What is carrying capacity and what limits population growth?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 4 of 7 WJEC exam series (2017 to 2025), 15 marks in total. This is a revision priority, not a prediction of your exam.

How does competition affect populations?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 13 marks in total. This is a revision priority, not a prediction of your exam.

How do ecologists sample organisms in a habitat?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 6 of 7 WJEC exam series (2017 to 2025), 23 marks in total. This is a revision priority, not a prediction of your exam.

How do transects show the distribution of organisms?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 13 marks in total. This is a revision priority, not a prediction of your exam.

Specified practical: investigating the distribution of seaweeds on a rocky shore

Equipment

Safety

Variables

Method, step by step

Results and analysis

Percentage cover of five seaweed species and height above the lowest tide at each quadrat along the transect

Kite diagram of percentage cover of five seaweed species along a rocky shore transect. Original BioWelsh diagram.

Profile of the rocky shore along the transect. Original BioWelsh graph.

What the results show

Worked calculations

Evaluation

Going further

Model answer: describe the method

Exam practice

What are ecosystems, habitats and communities?

What are gross and net primary production?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 9 marks in total. This is a revision priority, not a prediction of your exam.

How efficiently is energy transferred along food chains?

What do pyramids of numbers, biomass and energy show?

How does succession change bare rock into woodland?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 14 marks in total. This is a revision priority, not a prediction of your exam.

Why is the breakdown of dead organic matter important?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 9 marks in total. This is a revision priority, not a prediction of your exam.

How is carbon cycled in an ecosystem?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 12 marks in total. This is a revision priority, not a prediction of your exam.

How do human activities affect the carbon cycle and climate?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 12 marks in total. This is a revision priority, not a prediction of your exam.

What is the role of bacteria in the nitrogen cycle?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 19 marks in total. This is a revision priority, not a prediction of your exam.

How do farming practices affect the nitrogen cycle?

What causes eutrophication and algal blooms?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 18 marks in total. This is a revision priority, not a prediction of your exam.

How do the ideas in population size and ecosystems fit together?

Worksheets

Worksheet 1: 3.5 Populations and ecosystems (147 marks)

Check your understanding.

How do populations grow and change in size?

What is carrying capacity and what limits population growth?

How does competition affect populations?

How do ecologists sample organisms in a habitat?

How do transects show the distribution of organisms?

of five seaweed and height above the lowest tide at each along the

What are gross and net primary production?

How does succession change bare rock into woodland?

Why is the breakdown of dead organic matter important?

How is carbon cycled in an ecosystem?

How do human activities affect the carbon cycle and climate?

What is the role of bacteria in the nitrogen cycle?

What causes eutrophication and algal blooms?

Distance / m	Height / m	Channelled wrack / % cover	wrack / % cover	Bladder wrack / % cover	Serrated wrack / % cover	Oarweed / % cover
0	5.8	30	0	0	0	0
5	5.1	55	15	0	0	0
10	4.3	10	60	0	0	0
15	3.4	0	25	40	0	0
20	2.7	0	5	70	0	0
25	2.0	0	0	35	30	0
30	1.2	0	0	5	65	0
35	0.5	0	0	0	40	25
40	0.1	0	0	0	0	80